1. Field of the Invention
This invention relates to an optical modulation device. This invention also relates to an optical switch device.
2. Description of the Prior Art
Semiconductor lasers are widely used as light sources in optical communication systems. A typical way of modulating an optical signal is to control the injection current of a semiconductor laser in response to a modulating signal. However, it is generally difficult to operate a semiconductor laser at higher than several GHz frequencies by such direct modulation.
External electro-optic waveguide modulators using dielectric crystals can perform higher-frequency modulation. A typical electro-optic waveguide modulator includes a transmission line and an optical waveguide. The transmission line serves as a modulating electrode which enables a modulating signal to propagate on an electro-optical crystal. The optical waveguide extends near the transmission line. An electric field is induced around the modulating electrode in response to the modulating signal. The electro-optical effect causes the refractive indices of the optical waveguide to vary with the induced electric field. Accordingly, the phase of a light wave in the optical waveguide is varied in response to the modulating signal so that optical modulation is realized.
General electro-optical crystals such as an LiNbO.sub.3 crystal do not have large electro-optical coefficients. Thus, in an electro-optical modulator, it is important to effectively apply a modulating electric field to an optical waveguide to attain a good modulation efficiency.